This invention relates to semiconductor diodes and more specifically relates to a novel fast recovery diode (FRD) with ultra-soft turn off characteristics.
Fast recovery diodes are frequently used in motor control and power supply circuits having an inductive load. It is desirable that such FRDs have a xe2x80x9csoftxe2x80x9d recovery characteristic; that is, that its di/dt during turn off is limited to reduce the production of high voltage spikes in the inductive load circuit.
FRDs with soft recovery characteristics are described in U.S. Pat. No. 5,747,872 dated May 5, 1998. Those devices employ typical heavy metal doping, for example, gold or platinum, in selective areas of the semiconductor die to reduce lifetime in that area.
Heavy metal doping introduces complex processing steps and is not well controlled. Devices employing heavy metal doping also have a high negative temperature coefficient and are not easily connected in parallel. Further, the use of heavy metal in wafer fabrication facilities can adversely affect the processing operation in other portions of the facility.
It would be desirable to produce an ultra-soft recovery FRD diode in which lifetime in selected regions of the die is limited without the need for heavy metal doping.
In accordance with the invention, a novel ultra-soft FRD device and process of its manufacture is provided which excludes heavy metal from the process and employs only a helium implant and E-beam irradiation in the process, creating a desired gradient of lifetime of minority carriers in the silicon.
In particular, after the formation of the top surface of the die (or wafer containing plural die), helium is implanted into the front surface of the die to a given depth, preferably about 20 microns. This implant may be followed by an anneal at about 350xc2x0 C. This is followed by E-beam radiation over the full surface (to control the xe2x80x9ctailxe2x80x9d characteristic), followed by an anneal at about 300xc2x0 C.
The helium implant dose is in the range of about 5E9 to 2E11/cm2 and is preferably about 9E10 ions/cm2 and creates localized vacancies at a depth within the silicon, preferably slightly removed from the P/N junction of the device. A subsequent E-beam radiation is carried out at about 32kGray.
The final device is rated typically at 5 to 100 amperes at 1200 volts blocking. The device has a very soft recovery characteristic, making it ideal for use with motor control circuits.